SQL - Foreign Key

A foreign key is a key used to link two tables together. This is sometimes called a referencing key.
Primary key field from one table and insert it into the other table where it becomes a foreign key ie. Foreign Key is a column or a combination of columns whose values match a Primary Key in a different table.

The relationship between 2 tables matches the Primary Key in one of the tables with a Foreign Key in the second table.

If a table has a primary key defined on any field(s) then you can not have two records having the same value of that field(s).

Example:

Consider the structure of the two tables as follows:

CUSTOMERS table:

CREATE TABLE CUSTOMERS(
       ID   INT              NOT NULL,
       NAME VARCHAR (20)     NOT NULL,
       AGE  INT              NOT NULL,
       ADDRESS  CHAR (25) ,
       SALARY   DECIMAL (18, 2),       
       PRIMARY KEY (ID)
);

ORDERS table:

CREATE TABLE ORDERS (
       ID          INT        NOT NULL,
       DATE        DATETIME, 
       CUSTOMER_ID INT references CUSTOMERS(ID),
       AMOUNT     double,
       PRIMARY KEY (ID)
);

If ORDERS table has already been created, and the foreign key has not yet been, use the syntax for specifying a foreign key by altering a table.

ALTER TABLE ORDERS 

 ADD FOREIGN KEY (Customer_ID) REFERENCES CUSTOMERS (ID);

DROP a FOREIGN KEY Constraint:

To drop a FOREIGN KEY constraint, use the following SQL:

ALTER TABLE ORDERS
   DROP FOREIGN KEY;